1. Field of the Invention
The present invention relates to an optical disk apparatus, and more particularly to recording power adjustment for an optical disk when data recording with respect to a multilayer optical disk having a plurality of recording layers is performed.
2. Description of Related Art
Conventionally, multilayer optical disks having recording layers stacked in a multilayered form have been developed for the purpose of increasing storage capacity. In these multilayer optical disks, optimization of the recording power is required for each layer, and test writing of data is performed with respect to a test area provided for each layer to thereby select the optimum recording power.
On the other hand, optical disks in which a high power laser beam is irradiated onto a recording film in a crystalline state to cause the recording film to transit to an amorphous state for recording data, for example, suffer from a problem that, due to different light transmittances of the recording film between the amorphous state and the crystalline state, the recording state of a recording layer formed on the front side (a recording layer which is provided closer to the laser beam incident side) affects the data recording onto a recording layer on the back side (a recording layer which is provided further from the laser beam incident side). More specifically, if the recording layer on the front side is in a data recorded state, the transmittance of that recording layer is decreased. Therefore, in this case, the amount of laser beam reaching the recording layer on the back side is decreased compared to that when the recording layer on the front side is in a data unrecorded state. Consequently, the optimum recording power which is obtained with the recording layer on the front side in an unrecorded state is not sufficient.
In order to solve the above problem, Japanese Patent Laid-Open Publication No. 2004-171740 describes a technique in which a correction coefficient has been recorded on the recording layer on the back side in advance, and data is first recorded on the layer on the front side. When recording data on the recording layer on the back side, the correction coefficient is used to correct the recording power.
FIG. 5 shows a cross section of a multilayer optical disk 1 of the related art. The optical disk 1 includes a first recording layer 5 having a thickness of about 200 nm, a transparent separation layer 4 having a thickness of about 0.03mm, a second recording layer 3 having a thickness of about 100nm, and a protective layer 2, which are sequentially layered in this order on a polycarbonate substrate 6 having a thickness of about 1.1 mm. On the recording layers 3 and 5, an information track having a depth of about 20 nm and a width of about 0.2 μm is formed for tracking a laser beam at the time of recording and reproducing data. The laser beam is irradiated onto the optical disk from a surface on the side of the protective layer 2 as shown by an arrow in FIG. 5. Accordingly, data recording with respect to the first recording layer 5 is performed by the laser beam which has been transmitted through the second recording layer 3. The second recording layer 3 is formed by a material whose transmittance decreases by changing the state of the material from a crystalline state to an amorphous state by data recording. Correction information for correcting the laser beam intensity is recorded in a lead-in area 104 of the first recording layer 5. The correction information is a correction coefficient α used for correcting a decrease in the laser beam intensity caused by the decrease in the transmittance of the second recording layer 3 by data recording, and is determined as α=T1/T2 wherein T1 is a transmittance of the second recording layer in a data unrecorded state and T2 is a transmittance of the second recording layer in a data recorded state.
With the above structure, when the apparatus is activated, the correction coefficient α which is recorded in the lead-in area 104 of the first recording layer 5 is first read out, and then data is recorded onto the second recording layer 3 which is a recording layer on the front side. Then, when recording data on the first recording layer 5, which is a recording layer on the back side, the second recording layer 3 is already in a recorded state. Accordingly, after the laser beam intensity is corrected using the correction coefficient, data is then recorded on the first recording layer 5.
In the related art structure described above, however, it is necessary to first record data on the recording layer on the front side and then record data on the recording layer on the back side, as a result of which the order of recording is limited. More specifically, with the above structure, it is not possible to satisfy the need a user may have to first record data on the recording layer on the back side, or to first record data on the recording layer on the front side and then record data on either the recording layer on the front side or the recording layer on the back side in a desired order.
In this respect, the above-described publication further describes that it is possible to record data on the first recording layer 5 regardless of whether or not data has been recorded on the second recording layer 3, by setting the correction coefficient α such that the quality of a reproduced signal obtained from data recorded on the first recording layer 5 using the corrected laser beam when the second recording layer 3 is in the unrecorded state can satisfy a predetermined criterion. Even with this structure, however, setting of the correction coefficient a is complicated, and there still remains a problem that data recording with the optimum recording power cannot be performed even if the correction coefficient α can be set as described above, which leaves a problem of deterioration of the reproduced signal quality unsolved.